This is a Short Circuit

In the
previous part
, we saw the
operators for doing Boolean AND, OR and NOT.
In this part, we're going to look at one particular aspect
of these operators.
That feature is called
short-circuiting
.
The term short circuiting means that we don't evaluate the
second part of an AND or OR unless we really need to.
So for example, if we have an expression like:

if(a < b && c < d)

we don't bother evaluating the
c < d
part if
a
is not less than
b
since it doesn't matter
what
c
and
d
's relationship are; the condition is
false no matter what.
Similarly if we have

if(a < b || c < d)

we don't need to evaluate
c < d
if
a
is less than
b
since the OR will be true
regardless of what
c
and
d
are.

The importance of this feature is not just that it avoids
unnecessary computation.
The main thing is that if the second part of such an
expression has side effects, they won't always take place.
For example,

if(a < b && f(a, b))

will only execute the function
f()
if
a
is less than
b
.

Rule of Thumb

The main thing to get from this feature for now is the
following rule of thumb:

Unless you are intentionally taking advantage of the
short-circuiting when using
&&
or
||
,
don't put anything
with side-effects on the right hand side.